Routines

The control bitfield it builds, whose bits 0–1 and 5–7 are always 0:

Bit	When set
2	Down/back held
3	Up/forward held
4	Fire held

The control bitfield's two motion bits are forward and back along the bat's fixed edge.

Per-device reads, by input device value:

Device code	Device	Fire	Up (bit 3)	Down (bit 2)
0	Keyboard	Space	P	L
1	Kempston	port bit 4	port bit 3	port bit 2
2	Cursor	key 0	key 7	key 6
3	Sinclair	key 0	key 9	key 8

Each scheme tests its inputs and sets the output bit for each one. The field is zeroed at entry, so nothing is cleared mid-read. A pressed key on the keyboard and the two key-set joysticks reads as 0. The Kempston port reads a pressed input as 1.